1. Field of the Invention
The present invention relates to a semiconductor infrared detector, and more particularly to an infrared image pickup device comprising a plurality of infrared detector elements forming a photo-sensing substrate. The photo-sensing substrate of the invention provides a light shield layer specially prepared for optically isolating each detector element from adjacent detector elements and further isolating a signal processing substrate from the photo-sensing substrate. Therefore, the device is most suitable for an application in which high resolution is required.
2. Description of the Related Art
Many kinds of semiconductor materials are utilized as a photo-electric conversion element. Among them, semiconductor Hg.sub.1-x Cd.sub.x Te is known to have a narrow bandgap and to be a suitable material for an infrared detector. In a semiconductor image pickup device, a plurality of detector elements are arranged either one-dimensionally in a line or two-dimensionally in a matrix. In recent years, in order to obtain a higher resolution of the image pickup device, a distance between adjacent detector elements has been made smaller, which results in causing noise or cross talk problems.
An existing exemplary semiconductor image pickup device having a high sensitivity is illustrated in FIG. 1. The device Utilizes photovoltaic infrared detectors of compound semiconductor Hg.sub.1-x Cd.sub.x Te. The device is composed of two main portions of a photo-sensing substrate 30 and a signal processing substrate 32, and two substrates 30 and 32 are connected together by a plurality of connecting rods 13 (generally called a bump) therebetween.
With regard to an actual structure of the photo-sensing substrate 30, a plurality of infrared detector elements 1 are formed and buried in a CdTe substrate 5. Each detector element 1 comprises a P-type region 6 of compound semiconductor Hg.sub.1-x Cd.sub.x Te and an N-type region 7 of Hg.sub.1-x Cd.sub.x Te buried therein. With regard to the signal processing substrate 32, a plurality of diodes 14 are formed in a P-type silicon substrate 4, and the diode 14 is formed by a buried N-type region and a surrounding P-type silicon substrate region. Each N-type region 7 of the photo-sensing substrate 30 and each diode 14 of the signal processing substrate 32 are connected with a bump 13 of indium (In). The P-type regions 6 of the photo-sensing substrate 30 are connected together by a metal wiring layer 12, which forms a common electrode of the detector elements 1.
In the infrared image pickup device of the prior art, no light shield (or light absorber) is provided around an isolation region 2 between the adjacent detector elements 1, therefore, incident rays having the direction shown by arrows A cause irregular reflections from interfaces or the metal wiring layer 12 on the photo-sensing substrate 30. Some of the infrared rays penetrate through the photo-sensing substrate 30 and reach the surface of the signal processing substrate 32, and are reflected back therefrom. The above reflected rays, so-called stray rays, enter into the nearby detector element 1 and sensed therein as noise. All these phenomena deteriorate an S/N ratio and the aimed resolution of infrared image pickup device can not be obtained.
In order to reduce the generation of needless stray rays and to avoid the cross talk problem, the following two Japanese Unexamined Patents have been disclosed.
Japanese Unexamined Patent Publication SHO 63-296272 published Dec. 2, 1988 discloses the method, which a metal layer of gold (Au) 12 is formed on the surface of a photo-sensing substrate 30 except the surface of an N-type region 7 of the detector elements 1, and a substantial portion of a CdTe substrate 5 is etched away, which reduces the noise problem and further eliminates a thermal trouble caused by a difference in thermal expansion between the CdTe substrate 5 and the silicon substrate 4.
Japanese Unexamined Patent Publication HEI 12-01971 published Aug. 14, 1989 discloses that a photo-sensing substrate 30 comprises a buffer layer and infrared detector elements are not buried in a photo-sensing substrate 30 but each detector element has a mesa shape which is formed on the buffer layer and protrudes toward signal processing substrate 32. The surface of an isolation region 2, in other words, the bottom surface between detector elements (mesas) is covered with a light shield layer. The light shield layer is composed of two layers, i.e., a first layer of zinc sulphide (ZnS) and a second layer of chromium (Cr).